Databases, which collate data from a number of sources, including the U.S. Coast Guard, U.S. National Response Center, the U.S. Minerals Management Service, and the Oil Spill Intelligence Report.

As illustrated in Figure E-1, oil spillage dropped off significantly after 1991. This improvement followed the grounding of the Exxon Valdez in 1989, and the subsequent passage of the Oil Pollution Act of 1990 (OPA 90).

In recognition of the fundamental changes to the maritime industry that took place after the Exxon Valdez accident, spill data from 1990 onward were used as the basis for estimating the amount of oil entering U.S. waters from marine vessel accidents. Because complete spill data for Canadian and Mexican waters are not available, the estimated input from these waters was based on U.S. spill data adjusted for the relative tonnage moved through Canadian and Mexican waters as compared to U.S. waters.

Although effort is required to carefully evaluate data from the U.S. Coast Guard spill database to avoid double counting, the data were consistently collected during the 1990s and are considered reliable, particularly with regard to the larger spills. The database, which contains generally conservative estimates of outflow from all reported incidents, is assumed to establish the minimum estimate.

Table E-17 lists the amount of petroleum hydrocarbons spilled in the sea in U.S. marine waters from 1990 through 1999. A total of 1,745 spills of 0.34 tonnes (100 gallons) or more occurred during this ten year period, discharging some 9,111 tonnes (2,520,134 gallons) into marine waters. 175 of these spills were greater than 34 tonnes (10,000 gallons) in size, and these large spills accounted for about 87% of the total spillage.

Table E-18 shows the breakdown of the spillage by types of vessels and types of oil. Tankers and tank barges were responsible for 82% of the total spillage. Oil types were separated into four categories. Spillage by oil type was as follows: crude Oil (36%), heavy distillate (36%), light distillate (25%), and gasoline (3%).

Because a comprehensive spill database for Canadian and Mexican waters were not available, the spill volumes were estimated by adjusted U.S. figures by the relative movements of cargo. In 1997, approximately 715 million tonnes of crude oil and products were moved in U.S. international and coastwise trade (USACE, 1997b). In comparison, about 68 million tonnes were moved through Canadian ports (Statistics Canada, 1997), or 9.5 percent of the U.S. movements. Similarly, about 112 million tonnes of crude and products were moved through Mexican ports (BP World Statistics, 1997), or 15.7 percent of the U.S. movements. Inputs from accidental spills from tank vessels in Canada and Mexico were taken as 9.5 percent and 15.7 percent of the U.S. values respectively.

Canadian and Mexican dry cargo movements are approximately one-third and one-twentieth of the U.S. international and coastwise movements respectively. However, freighters are responsible for only 18 percent of the spillage from other vessels in U.S. waters. It was assumed that spills in Canadian water from other vessels equals 15 percent of the U.S. totals, and that spills in Mexican waters from other vessels equals 6 percent of the U.S. totals.

The U.S. Coast Guard database, which contains generally conservative estimates of outflow from all reported incidents, is assumed to establish the minimum estimate. Recognizing the completeness of the data, the spill quantities were increased by just 5 percent to obtain the best estimate, and further increased by 20 percent to obtain the maximum estimate. Results are summarized in Table E-19.

The recording of the location of spills was not as consistently maintained within the U.S. Coast Guard spill database. This data has been reviewed and summarized in Tables 2-2 through 2-6, in order to provide a sense of the distribution of spills within U.S. waters.

For this study, spill data from the Environmental Research Consulting database is applied. This database includes information gleaned from the International Maritime Organization, ITOPF, and other national and regional agencies. The international data excludes spills in North American waters. As shown in Table E-20, a total of 745,292 tonnes of oil spillage was recorded during the years 1990 through 1999,